The present invention relates to a battery pack having a desired output power and composed of a plurality of stacked battery modules each including a plurality of serially-connected cells, and a temperature control method and structure for this battery pack for equalizing the temperature of each battery module.
It is known in the prior art to constitute a battery module by connecting a plurality of cells in series to attain a larger output voltage, because discrete cells have only a small output voltage of about 1 V to 4 V. Such battery modules are further connected in series with each other for obtaining an even higher output voltage. Generally, a battery power source device having high output power is designed as a battery pack wherein a plurality of battery modules are arranged horizontally and stacked vertically in a holder case. Each of the large number of cells constituting the battery power source device is a rechargeable battery, and control is effected with respect to charging and discharging of the battery pack for obtaining the desired power.
Charging and discharging of rechargeable batteries involve generation of heat, and it is necessary to cool the battery pack in which a large number of cells are stacked, in order to prevent an excessive rise in temperature. On the contrary, in an application in a cold location where the temperature is below an adequate level for the battery, it may become necessary to heat the battery pack so as to prevent deterioration of the battery performance. It is also necessary to maintain all of the stacked cells at an equal temperature, because the battery properties change depending on the temperature of the battery.
For the temperature control of a battery pack, it is known in the art to employ a cooling structure as shown in FIG. 8, wherein a forced current of air is supplied from the underside of the holder case 10 of a battery pack 30 and passed between the plurality of battery modules 9 arranged horizontally and stacked vertically within the holder case 10.
However, in this cooling structure for a battery pack, as indicated by a broken line in the graph of FIG. 8, the air temperature Ta increases by the heat of battery modules 9 as the air flows upwards. Therefore, while the lowermost battery modules 9 are effectively cooled by the low-temperature air, those battery modules 9 located at higher tiers are poorly cooled because the temperature of the air has been increased due to heat exchange with the battery modules 9 of the lower tiers. As a result, the battery temperature TB is not uniform as indicated by the two dot chain line in FIG. 8, i.e., there is a large difference between the temperature TBB1 of the lowermost battery modules 9 and those TB3, TB5, TB7 of the other battery modules 9. Thus the battery modules 9 are not cooled uniformly.
It is an object of the present invention to provide a temperature control method and structure for a battery pack in which a plurality of battery modules are stacked, for maintaining each of the battery modules at an equal temperature.
In order to achieve the above-mentioned object, the present invention provides a temperature control method for a battery pack wherein a plurality of battery modules including a plurality of serially connected cells are arranged in parallel and stacked in piles within a holder case. A forced current of air is supplied into the holder case in a direction in which the battery modules are stacked for adjusting the temperatures of the battery modules. According to the invention, the battery modules located on an upstream side of the air current are covered with film tubes such as to form a gap between the battery module and the film tube, and the gap is made larger in battery modules which are located at a position with good heat exchange conditions.
With this temperature control method, the battery modules located on the upstream side of the air current are covered with film tubes to reduce the heat exchange efficiency thereof. The battery modules located upstream of the air current where good heat exchange is achieved are covered with film tubes with greater spaces between the surface of the battery modules and the film tubes. Accordingly, the heat exchange efficiency of the upstream battery modules provided with greater spaces between themselves and the film tubes is lowered. Such space between the battery module and the film tube is adjusted corresponding to the temperature of air current, whereby the temperatures of various battery modules are controlled to be equal. The battery characteristics dependent on temperature are thus equalized to enable the battery pack to exhibit high performance.
A temperature control structure according to the invention includes means for providing a forced current of air into the holder case in a direction in which the battery modules are stacked, and a plurality of film tubes respectively fitted on a plurality of battery modules located on an upstream side of the air current such as to form a gap between the battery module and the film tube. The gap is adjustable in size and made larger in battery modules which are located at a position with good heat exchange conditions.
The heat exchange efficiency of the battery modules covered with film tubes is lowered because of an air layer formed between the battery modules and the film tubes. Accordingly, the film tubes are fitted on the battery modules on the upstream side of the air current, whereby there is little difference in temperatures of a number of stacked battery modules.
Part of the battery modules is provided with a spacer, and the film tube is fitted on the spacer such that the battery module is covered with the film tube. The thickness of the spacer is varied for adjusting the size of the gap between the battery module and the film tube. The heat exchange efficiency of each battery module is thus adjusted by the thickness of the air layer formed between the battery module and the film tube.
The film tube may be composed of a resin film formed in a cylindrical shape. In this way, the thickness of the air layer, which affects the heat exchange efficiency of battery module, is simply adjusted by changing the diameter of the film tube.
The film tube may be formed of a resin film having a width greater than the circumference of a battery module. When wrapped into a cylindrical shape with the side edges being overlapped and joined together, it forms a tube having a diameter larger than the circumference of the battery module. In this way, reductions in cost and in space for storage and transfer are achieved in comparison to fabrication of resin films in cylindrical form.
Furthermore, the film tube may be formed of a resin film rolled into a cylindrical shape having a width such as to surround a battery module with a largest gap between the battery module and the film tube fitted thereon. In this way, film tubes having various different diameters are all formed of resin films of the same size.